Directions

In an 8-quart kettle combine potatoes with cold water to cover by 2 inches. Bring water to a boil and simmer potatoes until tender, 35 to 45 minutes.

While potatoes are cooking, in a heavy skillet cook leeks in 4 tablespoons butter over moderately low heat, stirring occasionally, until softened. Stir in thyme and salt and pepper to taste.

Drain potatoes in a colander and return to kettle. Dry potatoes over low heat, shaking kettle,1 minute. Cool potatoes just until they can be handled and peel. While potatoes are still warm, force through a ricer into a large bowl. In a small saucepan heat milk and cream until mixture just comes to a boil. Stir leeks and milk mixture into potatoes and season with salt and pepper. Spread potato mixture in a buttered 4-quart shallow baking dish. Chill potato mixture, covered, 1 day.

Preheat oven to 350°F.

Dot potato mixture with remaining 2 tablespoons butter and bake, covered with foil, in middle of oven until heated through and butter is melted, about 15 minutes.